There is an ongoing need for component miniaturization in radio wave communication devices. For example, smaller and more efficient components are needed for light-weight, hand-portable cellular telephones, wireless local area networks for linking computer systems within office buildings in a readily reconfigurable fashion, wristwatch- and credit-card-sized paging apparatus and other devices for promoting rapid, efficient and flexible voice and data communication.
Filters are needed for a variety of such communications applications wherein small size, light weight and high performance are simultaneously required. Increasing numbers of products seek to employ fixed spectral resources, often to achieve tasks not previously envisioned. Examples include cellular telephones, inter- and intra-facility computer-computer and/or computer-ancillary equipment linkages as well as a host of other, increasingly complex inter-personal and/or -equipment information sharing requirements. The desire to render increasingly complicated communications nodes portable and even hand-held and/or -portable and/or pocket-sized places extreme demands on filtering technology in the context of increasingly crowded radio frequency resources.
Acoustic wave filters provide filters meeting stringent performance requirements which are (i) extremely robust, (ii) readily mass produced, (iii) adjustment-free over the life of the unit and which (iv) sharply increase the performance to size ratio achievable in the frequency range extending from a few tens of MegaHertz to about several GigaHertz. However, need for low passband insertion loss simultaneously coupled with demand for high shape factor and high stopband attenuation pose filter design and performance requirements not easily met by a single acoustic wave filter alone.
One approach to satisfying these needs and demands is to cascade two or more acoustic wave filters. This approach realizes increased stopband signal rejection but requires additional matching components (e.g., inductors and/or capacitors) and also multiplies the volume and weight of the acoustic wave filters by the number of such filters cascaded. Matching components additionally incur major size and weight penalties because each transducer generally requires at least two matching components, each of which is at least as large as an acoustic wave filter die.
Another approach is to provide two or more such filters on a single substrate, wherein the filters are designed to have purely real impedances matched one to another without requiring intervening matching components. An example of such an approach is described in "Band Pass Filter Device", M. Hikita, U.S. Pat. No. 4,468,642 (Aug. 28, 1984), which is incorporated herein by reference. This approach has the advantages of improved performance, reduced size, weight and parts count and also provides improved manufacturability, coupled with greater ease of use. This approach has a first disadvantage of requiring the acoustic wave filter designer to restrict transducer impedances to a limited set of values in accordance with a rule: EQU N.congruent.C/k.sup.2, (1)
where N represents the number of effective finger pairs in the acoustic wave transducer. C is a constant given the value 1.5 in the prior art and the electromechanical coupling coefficient k.sup.2 is a material constant describing the acoustic wave substrate employed. A second disadvantage is that the two transducers must be substantially the same and must have substantially the same transfer function. Transducers designed in accordance with this rule have equal, purely real impedances at center frequency. Thus, transducers designed in accordance with this principle do not require additional matching components in order to be cascaded absent impedance mismatch loss penalties.
The prior art has addressed the insertion loss component of acoustic wave filter design through a number of approaches. These approaches include employing unidirectional transducers and also an arrangement wherein a number of input and output transducers are alternately disposed along an acoustic path to form a filter. The latter approach is described in some detail in "Surface Acoustic Wave Filter, Method and Apparatus", D. Allen, U.S. patent application No. 793,925 (filed Nov. 18, 1991) and also in "Acoustic Surface-Wave Bandpass Filter", M. Hikita, U.S. Pat. No. 4,492,940 (Jan. 8, 1985), which are hereby incorporated by reference.
The former approach is sometimes realized as a pair of unidirectional transducers coupled to an input, for example, on either side of a weighted, bidirectional transducer which is coupled to an output, for example. Both approaches are able to provide excellent in-band insertion loss characteristics and good shape factors. The disadvantage common to both approaches is that the stopband suppression is inadequate for many applications.
Thus, what are needed are practical, economical methods for, and apparatus employing, cascaded acoustic wave filters not requiring discrete matching components therebetween.